I'd just like to post this major message for all of you out there desparately looking for a sharp ultra-wide zoom and were discouraged from my first copy/report.
The 2nd copy of Sigma 10-20mm MUCH better!
I'll report more details as time permits but here is already a collection with pics from the new lens to quench some of your information-thirst.
The comparison to other wide-zooms is here.
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Last edited by Thomas on Fri Feb 04, 2011 9:52 pm, edited 5 times in total.

Here are the main facts from the catalog:
Size: 84x81mm = medium Weight: 470g = medium Optics: 14 elements in 10 groups = ok Closest focus distance/max.magnification: 0.24m / 1:6.7 (I got 1:6.9 in my test) --> Addendum: see what you still can do when the need arises here!
Filter-thread: 77mm = standard No IS = not so important AF-S, so works on D40/x-bodies , easy manual-focus override (just turn the focus ring) Covers APS-C (or smaller only = normal Comes with a lens-bag Price: around 460€ = not the cheapest --> Addendum: And the front- and rear lens-cap are still cr*p

Now to give you finally some of the testshots I've made, here is my favourite, that shows quite nicely what you can do with such an extreme lens, when you get close (enough) to your subject(s):
Taken at 12mm/f8.
So hear my warning (again): Even with a 12mm lens at f8 you do have a limited dof! Not everything will be automatically sharp from front to infinity. But hey, that's ok!

Now finally some serious test-shots to prove the qualities of this lens to you. Here we go, don't expect less than some Siemens-stars from me To make this procedure less painful for you I decided to hide the complete pics as links under the 100% crops. So if you don't believe me/your eyes just click through to the original. Apart from my feet there is also the other Siemens-star in the lower left corner for you to check that the perpendicular adjustment on my test shots was ok.
Following are the 100% crops first centre, then the upper right corner. All shots at maximum aperture (=wide-open) first row at 10mm:

...followed by 15mm (there are some tests saying the range in between is worse than at both ends of the focal-length)

...and finally 20mm. Enjoy!

When measuring the diameter of the grey disks at the centre of the Siemens-star, I normally go for 200% magnification and divide the result by two, just to make it easier for my eyes to judge! So what do we get here:
10mm centre 6 mm, corner 7-9 mm
15mm centre 7 mm, corner 8-9 mm
20mm centre 7 mm, corner 8-9 mm
At the corners there is a slight astigmatism, meaning that the sharpness depents on the direction of lines.
The obvious loss of contrast in the corners at 10mm gets a little better by stopping down to f8 or so, but not really up to the centre-contrast.
As you can see, the results are pretty impressive across the focal range. MUCH better than the results from my first copy. And this clearly shows in the pics you take with lens.

Expect some white-dwarf results soon...

And let me show you some (slightly tweaked, but no HDR) shot of what I love very much: Sky, clouds and sun!
Dark Doom

First of all a warning: you can get that ugly brownish stripes that the Sigma 12-24mm exhibited, look closely at the bush to the left:
But it seems somewhat subdued compared to the effect on the Sigma 12-24mm here.

This is, why a more "scientific" approach is needed to tell the weaknesses of the Sigma 12-24mm and the 10-20mm apart. If you already know my test-technology you may proceed, otherwise, a quick read over here could cast some light (sic!) on how I "do the white dwarf" What I show is some shot approaching worst-case and some typical shot. If you want to analyse the situation for different settings of focal-length and/or aperture any further, feel free to do so. The respective photos are over here, or filter my pictures for "whitedwarf" to have a look at the whole caboodle.

The first is at 10mm f8, the second at 20mm f5.6

As you can see, the problem can be reproduced in the "lab" but is by far not as troublesome as with the Sigma 12-24mm. But it is uglier than with the Tokina 12-24mm or the Tamron 11-18mm which is the best in this respect.

And one final challenge for all those pixel-peepers out there, here is the ultimate test-shot to judge corner sharpness, geometry and color-fringing all in one simple picture!

Ah well, I forgot that I promised you to also test close-up performance, which is seldom done in reviews...
So if you want to have a look for yourself, just surf over here or there for two illuminating Siemens-stars shot from the same distance at 10mm and 20mm (giving approx 1:20 resp. 1:10 magnification) wide open.
Taking out my trusty old ruler I measure the following:
at 10mm: 7-8mm centre
at 20mm: 9-10mm centre and 10-12mm at the borders
So you can clearly see that close-up performance is worse than at greater distances esp. at the long end and that there is some pronounced deterioration at the borders at 20mm. Interestingly there is some "veiling" effect on the smaller Siemens-stars in the corners. There seems to be a clear loss in contrast at around 20mm diameter (like a veil) but the lines are still visible up to the diameters I gave you above.
So don't expect miracles of this lens in close-up situations at the borders. The centre at the wide end is still very good though.

Jason, I'm not sure. From the outside, you could not distinguish the first and the second copy. But even at the time of testing the first copy there were reports that some people got good copies and others complained. So I just put it down to sloppy quality control at Sigma. The bad thing is, you'll never know: maybe QC is still sloppy and I just turned out to be lucky, or Sigma tightened it's QC-system and more lenses are now ok! If the latter is the case you may want to look at copies with serial-nos. above 2393700.
My advice: It's best to buy from a dealer where you can give the lens back within the first few days in case you happen to get a bad copy.